Process for forming shaped group II-VI semiconductor nanocrystals, and product formed using process

ABSTRACT

A process for the formation of shaped Group II-VI semiconductor nanocrystals comprises contacting the semiconductor nanocrystal precursors with a liquid media comprising a binary mixture of phosphorus-containing organic surfactants capable of promoting the growth of either spherical semiconductor nanocrystals or rod-like semiconductor nanocrystals, whereby the shape of the semiconductor nanocrystals formed in said binary mixture of surfactants is controlled by adjusting the ratio of the surfactants in the binary mixture.

CROSS-REFERENCE TO RELATED APPLICATION

This application is related to U.S. patent application Ser. No.09/499,096, entitled PROCESS FOR FORMING SHAPED GROUP III-VSEMICONDUCTOR NANOCRYSTALS, AND PRODUCT FORMED USING PROCESS, which wasfiled on Feb. 4, 2000, and is assigned to the assignee of thisinvention.

The invention described herein arose in the course of, or under,Contract No. DE-AC03-SF00098 between the United States Department ofEnergy and the University of California for the operation of the ErnestOrlando Lawrence Berkeley National Laboratory. The Government may haverights to the invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a process for making shaped semiconductornanocrystals and a resulting product therefrom. More particularly, thisinvention relates to a process for controlling the shape of Group II-VIsemiconductor nanocrystals during the formation of same.

2. Description of the Related Art

Semiconductor nanocrystals, such as Group II-VI nanocrystals are formedby dissolving a Group II precursor and a Group VI precursor in a solventand then applying heat to the solvent and the precursors therein. Forexample, Group II-VI semiconductor nanocrystals may be formed bydissolving a dialkyl of the Group II metal and a Group VI powder in atrialkyl phosphine solvent at ambient temperature, and then injectingthe mixture into a heated (340° C.-360° C.) bath of tri-octyl phosphineoxide (TOPO).

While the just described process is capable of producing Group II-VIsemiconductor nanocrystals, the results can be somewhat erratic in termsof average particle size and size distribution. While it is not certainwhy the process is not always reproducible, it is believed thatimpurities in the technical grade (90% pure) TOPO may be adverselyinfluencing the reaction. However, substitution of pure TOPO for thetechnical grade TOPO has also been unsatisfactory, particularly whencontrol of the shape of the particle growth is also desired, apparentlybecause the pure TOPO binds too weakly to the growing crystallites andonly weakly associates with the Group II metal to act as a growthretardant, resulting in the growth of spheres rather than any otherdesired shapes. Apparently, the presence of impurities in the technicalgrade TOPO result in the erratic success of Group II-VI semiconductornanocrystal growth in technical grade TOPO.

The growth of rod-like semiconductor crystals has been reported by W. Z.Wang et al. in “Synthesis and Characterization of MSe (M=Zn, Cd)nanorods by a New Solvothermal Method”, Inorganic ChemistryCommunications 1999 Mar, Vol. 2, N3:83-85. However, the rod-shapedcrystals are out of the confinement region, i.e., are not of nanocrystaldimensions.

Alivisatos et al. U.S. Pat. No. 5,505,928, by one of us with another,and assigned to the assignee of this invention, and the subject matterof which is hereby incorporated by reference, describes a process forforming Group Ill-V semiconductor nanocrystals wherein size control isachieved through use of a crystallite growth terminator which controlsthe size of the growing crystals. Crystallite growth terminators aresaid to include a nitrogen-containing or a phosphorus-containing polarorganic solvent having an unshared pair of electrons. The patent statesthat this growth terminator can complex with the metal and bind to it,thereby presenting a surface which will prevent further crystal growth.

Since Group II-VI semiconductor nanocrystals are of interest for use inoptical displays, as well as in biological applications, it would bedesirable to provide a process for control of shape as well as sizegrowth of such Group II-VI semiconductor nanocrystals wherein the shape(aspect ratio), as well as the particle size, growth rate, and particlesize distribution, can be reproducibly controlled, whereby, for example,spheres or rods of semiconductor nanocrystals of controlled dimensionscould be formed in a controllable and repeatable manner.

SUMMARY OF THE INVENTION

In accordance with the invention, a process for the formation of shapedGroup II-VI semiconductor nanocrystals comprises contacting a solutionof the semiconductor nanocrystal precursors with a liquid mediacomprising a binary surfactant mixture capable of promoting the growthof either spherical semiconductor nanocrystals or rod-like semiconductornanocrystals, whereby the shape of the semiconductor nanocrystals formedin the binary mixture of surfactants is controlled by adjusting theratio of the surfactants in the binary mixture.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow sheet describing the process of the invention.

FIG. 2 depicts a rod-shaped semiconductor nanocrystal formed using theprocess of the invention.

DETAILED DESCRIPTION OF THE INVENTION a. Introduction

The invention comprises a process for the formation of shaped GroupII-VI semiconductor nanocrystals comprises contacting a solution of thesemiconductor nanocrystal precursors with a liquid media comprising abinary surfactant mixture capable of promoting the growth of eitherspherical nanocrystals or rod-like nanocrystals, whereby the shape ofthe semiconductor nanocrystals formed in said binary mixture ofsurfactants is controlled by adjusting the ratio of the surfactants inthe binary mixture.

b. Semiconductor Nanocrystal Constituents

The precursors used to form the shaped Group II-VI semiconductornanocrystals are particular Group II and Group VI precursors used toform Group II-VI semiconductor nanocrystals. Thus, by use of the term“semiconductor nanocrystal” is meant a nanocrystal comprising thereaction of at least one particular Group II metal with at least oneparticular Group VI element. It should be noted here that the expression“at least one” is used in recognition of the fact that, for example, aparticular Group II metal precursor may be reacted with more than oneparticular Group VI precursor, and vice versa. The purity of theparticular Group II and Group VI precursors should be at least about99.9 wt. % to avoid introduction of impurities into the reaction whichcould have an effect on control of the growth of the semiconductornanocrystals.

By use of the term “particular Group II metal” is meant zinc, cadmium,or mercury (Zn, Cd, or Hg). By use of the term “particular Group VIelement” is meant sulfur, selenium, or tellurium (S, Se, or Te).

c. Semiconductor Nanocrystal Sizes

By use of the term “nanocrystal” is meant a small crystal havingdimensions not exceeding about 20 nm, and preferably not exceeding about10 nm, along at least 2 axes of the crystal. It should be noted that theabove definition of the term “nanocrystal” is somewhat different thanprevious definitions of the term in that this definition takes intoaccount the possibility of some extended growth of the nanocrystal alongthe third axis of the crystal while maintaining the very smalldimensions of the other two axes, resulting in the possibility of growthof a rod-like shaped semiconductor nanocrystal or quantum rod of verysmall cross-section (diameter).

d. Binary Surfactant Mixture

By use herein of the term “binary surfactant mixture” is meant a highboiling liquid mixture of two non-reactive organic surfactants in whichthe reaction to form the Group II-VI semiconductor nanocrystals takesplace. The binary mixture of non-reactive organic surfactants is capableof promoting the growth of either spherical semiconductor nanocrystalsor rod-like shaped semiconductor nanocrystals, depending on the ratio ofthe surfactants.

The binary surfactant mixture referred to above as a high boiling liquidmixture should have a boiling point sufficient to permit it (the binarysurfactant mixture) to be maintained at a temperature high enough topermit reaction between the Group II and Group VI precursors to form thedesired semiconductor nanocrystals. Typically, the binary surfactantmixture has a boiling point sufficiently high to permit the binarysurfactant mixture to be maintained at a temperature of from at leastabout 200° C., but not exceeding about 400° C.

The first organic surfactant in the binary mixture may be defined as anyliquid surfactant capable of being heated to such crystal-growingtemperatures and which further promotes growth of spherically shapedsemiconductor nanocrystals with increased percentage of the firstsurfactant in the ratio of the surfactants in the binary mixture, i.e.,when a sufficient amount of the first surfactant is present in thebinary mixture of surfactants.

Preferably the first liquid surfactant in the binary mixture willcomprise a phosphorus-containing surfactant capable of withstanding suchcrystal-growing temperatures. Examples of such firstphosphorus-containing liquid surfactants include liquid surfactants suchas 3-18 carbon trialkyl phosphines (e.g., tributyl phosphine), or 3-18carbon trialkyl phosphine oxides (e.g., trioctyl phosphine oxide—TOPO).To avoid the introduction of impurities which may have an unpredictableeffect on the reaction, the organic surfactant capable of promoting theformation of spherical semiconductor nanocrystals should preferably havea purity of at least 99 wt. %.

The second organic surfactant in the binary mixture may defined as anyliquid surfactant capable of being heated to such crystal-growingtemperatures and which further promotes growth of rod-like shapedsemiconductor nanocrystals with increased percentage of the secondsurfactant in the ratio of the surfactants in the binary mixture, i.e.,when a sufficient amount of the second surfactant is present in thebinary mixture of surfactants.

Preferably the second liquid surfactant capable of promoting the growthof rod-like shaped semiconductor nanocrystals will also comprise aphosphorus-containing surfactant capable of withstanding suchcrystal-growing temperatures.

Preferably, the second organic surfactant comprises anorganic-substituted acid, or acid salt surfactant containing phosphorussuch as, for example, phosphonic and phosphinic acids. Such phosphonicacids useful in the practice of the invention have the general formulaROP(OH)₂, where R is a 3-18 carbon (but preferably a 6-10 carbon)organic group such as an alkyl or aryl group, and can be shown by thetwo resonance forms:

Phosphinic acids useful in the practice of the invention may includemono and diphosphinic acids having the general formulaR′R_(x)H_((1-x))POOH, where R and R′ are the same or different 3-18carbon (but preferably 6-10 carbon) organic groups such as alkyl or arylgroups, and x is 0-1. The most significant resonance forms for the monophosphinic acid are shown as follows:

The most significant resonance forms for the diphosphinic acid are shownas follows:

Typically, the second organic surfactant will comprise a 6-10 carbonalkyl phosphonic acid, e.g., hexyl phosphonic acid. To avoid introducinguncontrollable impurities into the reaction which could have an effecton control of the growth of the semiconductor nanocrystals, the purityof the second organic surfactant in the binary mixture should be atleast 95 wt. %

e. Concentration of Respective Surfactants in Binary Mixture

While we do not wish to be bound by any particular theories as to howthe surfactants operate in the reaction to control both the size andshape of the semiconductor nanocrystals, it is believed that one or bothof the surfactants in the binary mixture of surfactants binds orassociates with the metal precursor, i.e., the Group II metal, to slowthe initial growth of the semiconductor crystal, thus resulting in theformation or growth of the desired nanocrystals, rather than macrocrystals. It is also believed that the second surfactant in the binarymixture further preferably binds to certain faces of the growingcrystallites, depending upon the concentration of the second surfactantin the binary mixture, to control the shape of the growing crystalliteas well, i.e., to control whether or not spheres or rods ofsemiconductor nanocrystals will form.

With the above in mind, it has been found the presence of less than acertain weight percentage of the second surfactant in the binary mixturewill result in the formation and growth of generally sphericalnanocrystals. Thus, for example, when tri-octyl phosphine oxide (TOPO)was used as the surfactant in the binary mixture as the first surfactantto promote the formation of spherical semiconductor nanocrystals andhexyl phosphonic acid was used as the second surfactant in the binarymixture, concentrations of 1.5 wt. % (3.42 molar %) and 3 wt. % (6.71molar %) hexyl phosphonic acid in the binary mixture (i.e., less thanabout 5 weight % (10.91 molar %), with the balance TOPO, results information of generally spherical nanocrystals, while respectiveconcentrations of 5 wt. % (10.91 molar %), 10 wt. % (20.54 molar %), and20 wt. % (37 molar %) hexyl phosphinic acid in the binary mixture, withthe balance comprising TOPO, reproducibly produce rod-like shapedsemiconductor nanocrystals.

Thus, to grow nanocrystals having two axes of very small length, with anenlarged third diameter, i.e., a rod of very small cross sectional area,larger concentrations of the second surfactant must be used in thebinary mixture of surfactants. Exact concentrations of a particularsecond surfactant necessary to achieve a rod-like semiconductornanocrystal rather than a spherical nanocrystal will depend upon theparticular first and second surfactants used and the particular Group IIand Group VI precursors, and may be empirically determined for eachbinary mixture of surfactants and particular precursors.

f. Temperatures of Binary Mixture of Surfactants Before and AfterIntroduction of Precursor Solution

During the initial introduction of the dissolved Group II and Group VIprecursors, the solution of precursors is injected into the high boilingbinary mixture of surfactants comprising the reaction media while thebinary mixture is maintained at a first temperature which results ininstantaneous nucleation of seed crystals. This temperature, forexample, when cadmium selenide (CdSe) nanocrystals are being formed in abinary mixture of TOPO and hexyl phosphonic acid, may be about 360° C.While initiation of the crystal growth at this higher first temperatureis important to the preferential growth of nanocrystals of theprecursors rather than macro crystals, it is equally important that thecontinued nucleation of such seed crystals be arrested in favor ofgrowth of the already formed seed crystals. If the continued nucleationis not stopped, the processes of nucleation and growth will coexist fora certain amount of time, resulting in a final large distribution ofboth aspect ratios and sizes for the nanocrystals.

Therefore, the solution of precursors is injected, preferably as a coldsolution, e.g., at room temperature, so that immediately after theinjection the temperature of the high boiling binary mixture ofsurfactants drops to a second temperature of lower value which is keptconstant during the nanocrystal growth. This temperature, for example,when cadmium selenide (CdSe) semiconductor nanocrystals are formed in abinary solution of TOPO and hexyl phosphonic acid, may range from about250° C. to about 300° C. if the initial temperature ranges from 280° C.to 360° C.

Subsequent nanocrystal growth is then stopped by a further reduction ofthe temperature to below the temperature at which nanocrystal growthoccurs. Since this temperature may vary from precursor to precursor, andprecise control of the time period for crystal growth is desirable,cessation of the crystal growth may be conveniently accomplished byrapidly reducing the temperature to ambient temperature or even lower,e.g., down to about 25° C. or lower, e.g., by removing the heatingmantle, and can be more rapid if the walls of the reactor are cooledwith a stream of air.

g. Concentration of Precursors in Solvent

The particular Group II and Group VI precursors may be dissolved in anyorganic liquid compatible with the binary mixture of surfactants, suchas any polar organic solvent such as a trialkyl phosphine, e.g.,tributyl phosphine. The precursors may be dissolved in the same solventor may be dissolved separately to form two solutions. Advantageously,for purposes of subsequently injecting the dissolved precursors into theheated binary mixture of surfactants, it is preferred to form a singlesolution containing all the dissolved precursors therein.

The solution or solutions containing the dissolved precursors aremaintained at a temperature below crystal growth temperature(conveniently at ambient temperature or lower), while the binary mixtureof surfactants is heated to the first (nucleation) temperature. Thesolution (or solutions) containing the precursors is then injected intothe binary mixture liquid media at a high rate of speed, for examplethrough a needle or a pressurized nozzle, to thereby rapidly heat theprecursors up to the first nucleation temperature. The concentration ofthe precursors in the high boiling binary mixture liquid media shouldinitially be about 0.18 M at the time of initial injection of theprecursor solution into the high boiling binary mixture liquid media.

h. Time of Reaction to Grow Semiconductor Nanocrystal Rods

The time of the reaction to grow the Group II-VI semiconductornanocrystal rods will vary with the type of semiconductor precursors,the composition and temperature of the binary mixture of surfactantsconstituting the liquid media in which the crystals are growing, and theconcentration of the precursors in the binary mixture of surfactants, aswell as the desired length and aspect ratio of the semiconductornanocrystal rods. For example, for the growth of CdSe semiconductor rodshaving an average length of 150 nm, and an aspect ratio of 5, in abinary mixture liquid media maintained at a temperature of about 300°C., and at a precursor concentration of 0.18 M, the desiredsemiconductor nanocrystals may be grown in a period of from about 300seconds to about 600 seconds. If spherical semiconductor nanocrystalgrowth is desired, the time periods may be extended to as long asseveral hours.

The following will serve to further illustrate the practice of theinvention.

To form CdSe semiconductor nanocrystals, a solution of Cd and Seprecursors is prepared by dissolving 0.82 grams of Cd(CH₃)₂ and 0.4grams of Se powder into 15.3 grams of tributyl phosphine. 2 ml. of thissolution (kept at −10° C.) was then quickly injected into 4 grams of aheated bath comprising a binary mixture of (99% pure) tri-octylphosphine oxide (TOPO) and 17 molar % hexyl phosphonic acid (HPA) whichhad been preheated to a temperature of about 360° C. After theinjection, the temperature of the binary mixture bath drops toapproximately 300° C. and is kept at this temperature for about 5-10minutes for the fast growth of rod-like semiconductor nanocrystals.After this time period, the binary mixture bath is rapidly cooled. Thesame procedure may be used to form spherical semiconductor nanocrystalsby reducing the concentration of the HPA down to, for example, about 4molar % and increasing the reaction time at 300° C. up to several hours.

The reaction was repeated several times with the concentration of theHPA surfactant in the binary mixture bath respectively at 1.5 weight %(3.42 molar %), 3.0 weight % (6.61 molar %), 5.0 weight % (10.91 molar%), 8.0 weight % (16.83 molar %), 10.0 weight % (20.54 molar %), and20.0 weight % (36.78 molar %), with the reaction time extended toseveral hours for the reactions using 1.5 and 3.0 weight % HPA. Thesemiconductor nanocrystals recovered from each batch were examined undertransmission electron microscopy (TEM) and powder x-ray diffraction(XRD). The semiconductor nanocrystals grown in the baths respectivelycontaining 1.5 and 3.0 weight % of the HPA growth control agent weregenerally spherical in shape, while the semiconductor nanocrystals grownin the baths containing 5.0, 8.0. 10.0, and 20.0 weight % HPA hadrod-shaped morphologies. Both measurements showed that the long axis ofthe rods along the c-axis was consistent with a wurtzite structure.Perpendicular to the c-axis, the semiconductor nanocrystal rods appearas faceted hexagons. Similar results may be obtained substituting otherparticular Group II metals (Zn or Hg) for Cd and/or substituting otherparticular Group VI precursors (S or Te) for Se.

Having thus described the invention what is claimed is:
 1. A process forthe formation of Group II-VI semiconductor nanocrystals comprisesintroducing the precursors into a heated binary mixture of first andsecond different surfactants capable of promoting the growth of eitherspherical semiconductor nanocrystals or rod-like semiconductornanocrystals, whereby the shape of said semiconductor nanocrystalsformed in said binary mixture of surfactants is capable of beingcontrolled by adjusting the ratio of said first and second surfactantsin said binary mixture.
 2. The process of claim 1 wherein said heatedbinary mixture of surfactants comprises two differentphosphorus-containing surfactants.
 3. The process of claim 2 whereinsaid binary mixture of phosphorus-containing surfactants is maintainedat a temperature sufficiently high to promote said growth of saidsemiconductor nanocrystals.
 4. The process of claim 2 wherein said ratioof said first and said second phosphorus-containing surfactants in saidheated binary mixture of surfactants permits growth of substantiallyspherical semiconductor nanocrystals in said binary mixture.
 5. Theprocess of claim 2 wherein said ratio of said first and said secondphosphorus-containing surfactants in said heated binary mixture ofsurfactants permits growth of rod-like semiconductor nanocrystals insaid binary mixture.
 6. The process of claim 5 wherein said secondphosphorus-containing surfactant in said binary mixture of surfactantscomprises a phosphorus-containing organic acid.
 7. The process of claim6 wherein said phosphorus-containing organic acid surfactant comprises aphosphonic acid surfactant having the formula ROP(OH)₂, where R is a3-18 carbon organic group.
 8. The process of claim 6 wherein saidphosphorus-containing organic acid surfactant is a phosphinic acidhaving the formula R′R_(x)H_((1-x))POOH, where R and R′ are the same ordifferent 3-18 carbon organic groups and x is 0-1.
 9. A process forforming shaped Group II-VI semiconductor nanocrystals in a heated binarymixture of surfactants which comprises: a) forming one or moresolutions, each comprising one or more Group II and/or Group VIsemiconductor nanocrystal precursors dissolved in a liquid which is asolvent for said one or more semiconductor precursors; b) forming abinary mixture of high boiling organic surfactants comprising: i) afirst surfactant; and ii) a second surfactant; c) adjusting the ratio ofsaid first and second surfactants in said binary mixture to control theshape of said semiconductor nanocrystals to be formed therein; d)heating said binary mixture of surfactants to a temperature at whichsaid Group II and Group VI precursors will react in said heated binarymixture of surfactants to form said Group II-VI semiconductornanocrystals; e) adding said one or more solutions of Group II and GroupVI semiconductor nanocrystal precursors to said heated binary mixture ofsurfactants to form said semiconductor nanocrystals; and f) subsequentlyreducing the temperature of said heated binary mixture of surfactantscontaining said semiconductor nanocrystals to stop said growth of saidsemiconductor nanocrystals.
 10. The process of claim 9 wherein said stepof forming said one or more solutions, each comprising one or more GroupII and Group VI semiconductor nanocrystal precursors dissolved in aliquid which is a solvent for said one or more semiconductor precursorsfurther comprises: (a) forming a first solution containing at least oneGroup II metal selected from the group consisting of Zn, Cd, and Hg; and(b) forming a second solution containing at least one Group VI elementselected from the group consisting of S, Se, and Te.
 11. The process ofclaim 9 wherein said step of forming said one or more solutions, eachcomprising one or more Group II and Group VI semiconductor nanocrystalprecursors dissolved in a liquid which is a solvent for said one or moresemiconductor precursors further comprises forming a single solutioncontaining a) at least one or more Group II metals, or compoundscontaining Group II metals, selected from the group consisting of Zn,Cd, and Hg; and (b) at least one or more Group VI elements, or compoundscontaining Group VI elements, selected from the group consisting of S,Se, and Te.
 12. The process of claim 9 wherein at least one of said oneor more solutions comprises a) a particular Group II metal; and b) aparticular Group VI element.
 13. The process of claim 9 wherein: a) saidheating step is carried out until said binary mixture of surfactantsreaches a temperature at which said semiconductor nanocrystals willnucleate when said one or more solutions are added to said binarymixture; b) said nucleation temperature is lowered to a crystal growthtemperature after said step of adding said solution to said heatedbinary mixture of surfactants; and c) said heated binary mixture ofsurfactants containing said one or more solutions is maintained at saidcrystal growth temperature until said step of reducing the temperatureof said mixture containing said solution sufficiently to stop formationof said semiconductor nanocrystals.
 14. The process of claim 9 whereinsaid concentration of said first surfactant in said binary mixture ofsurfactants is increased sufficiently to permit growth of substantiallyspherical semiconductor nanocrystals in said binary mixture ofsurfactants.
 15. The process of claim 9 wherein said concentration ofsaid second surfactant in said binary mixture of surfactants isincreased sufficiently to permit growth of rod-like semiconductornanocrystals in said binary mixture of surfactants.
 16. The process ofclaim 9 wherein said heated binary mixture of surfactants comprises twodifferent phosphorus-containing surfactants.
 17. The process of claim 16wherein said second phosphorus-containing surfactant in said binarymixture of surfactants comprises a phosphorus-containing organic acid.18. The process of claim 17 wherein said phosphorus-containing organicacid second surfactant comprises a phosphonic acid surfactant having theformula ROP(OH)₂, where R is a 3-18 carbon organic group.
 19. Theprocess of claim 17 wherein said phosphorus-containing organic acidsecond surfactant is a phosphinic acid having the formulaR′R_(x)H_((1-x))POOH, where R and R′ are the same or different 3-18carbon organic groups and x is 0-1.
 20. A process for forming shapedGroup II-VI semiconductor nanocrystals in a heated binary mixture ofsurfactants which comprises: a) forming a solution comprising: i) afirst semiconductor precursor comprising one or more particular Group IImetals, or compounds containing one or more particular Group II metals;ii) a second semiconductor precursor comprising one or more particularGroup VI elements, or compounds containing one or more particular GroupVI elements; and iii) a liquid which is a solvent for said first andsecond precursors; b) forming a binary mixture of high boilingphosphorus-containing organic surfactants comprising: i) a firstsurfactant capable of promoting growth of spherical semiconductornanocrystals when a sufficient amount of said surfactant is present insaid binary mixture of surfactants; and ii) a second surfactant capableof promoting growth of rod-like shaped semiconductor nanocrystals when asufficient amount of said surfactant is present in said binary mixtureof surfactants; c) adjusting the ratio of said first and secondsurfactants in said binary mixture to control the shape of saidsemiconductor nanocrystals to be formed therein; d) heating said binarymixture of phosphorus-containing organic surfactants to a temperaturesufficient to permit formation of Group II-VI semiconductor nanocrystalstherein; e) adding said solution of Group II and Group VI semiconductornanocrystal precursors to said heated binary mixture of surfactants toform said semiconductor nanocrystals; and f) subsequently reducing thetemperature of said heated binary mixture of surfactants containing saidsemiconductor nanocrystals to stop said growth of said semiconductornanocrystals.
 21. The process of claim 20 wherein: a) said heating stepis carried out until said binary mixture of surfactants reaches atemperature at which said semiconductor nanocrystals will nucleate whensaid one or more solutions are added to said binary mixture; b) saidnucleation temperature is lowered to a crystal growth temperature duringor after said step of adding said solution to said heated binary mixtureof surfactants; and c) said heated binary mixture of surfactantscontaining said one or more solutions is maintained at said crystalgrowth temperature until said step of reducing the temperature of saidmixture containing said solution sufficiently to stop formation of saidsemiconductor nanocrystals.
 22. The process of claim 21 wherein saidnucleation temperature is lowered to a crystal growth temperature duringsaid step of adding said solution of Group II and Group VI semiconductornanocrystal precursors to said heated binary mixture of surfactants byadding to said binary mixture of surfactants said solution of Group IIand Group VI semiconductor nanocrystal precursors at a temperaturesufficiently lower than the temperature of said binary surfactantmixture to thereby cool said binary mixture of surfactants down to saidcrystal growth temperature as said solution of Group II and Group VIsemiconductor nanocrystal precursors is added to said heated binarymixture.
 23. The process of claim 20 wherein said Group II metal isselected from the group consisting of Zn, Cd, and Hg; and said Group VIelement is selected from the group consisting of S, Se, and Te.
 24. Theprocess of claim 20 wherein said liquid which is a solvent for saidfirst and second precursors comprises a polar organic solvent.
 25. Theprocess of claim 23 wherein said solvent comprises a trialkyl phosphine.26. The process of claim 20 wherein said first surfactant in said binarymixture of surfactants is selected from the group consisting of a 3-18carbon tri-alkyl phosphine and a 3-18 carbon tri-alkyl phosphine oxide.27. The process of claim 20 wherein said second surfactant in saidbinary mixture of surfactants comprises a phosphorus-containing organicacid.
 28. The process of claim 27 wherein said phosphorus-containingorganic acid second surfactant in said binary mixture comprisesphosphonic acid having the formula ROP(OH)₂, where R is a 3-18 carbonorganic group.
 29. The process of claim 28 wherein said phosphonic acidsecond surfactant comprises hexyl-phosphonic acid.
 30. The process ofclaim 27 wherein said phosphorus-containing organic acid secondsurfactant is a phosphinic acid having the formula R′R_(x)H_((1-x))POOH,where R and R′ are the same or different 3-18 carbon organic groups andx is 0-1.
 31. A process for forming shaped CdSe semiconductornanocrystals in a heated binary mixture of surfactants which comprises:a) forming a solution comprising: i) a first semiconductor precursorcomprising Cd metal, and/or one or more compounds containing Cd metal;ii) a second semiconductor precursor comprising Se, and/or one or morecompounds containing Se; and iii) a liquid which is a solvent for saidfirst and second precursors; b) forming a binary mixture of high boilingphosphorus-containing organic surfactants comprising: i) a firstphosphorus-containing surfactant capable of promoting growth ofspherical semiconductor nanocrystals with increased percentage of saidfirst surfactant in the ratio of said surfactants in said binary mixtureof surfactants; and ii) a second surfactant capable of promoting growthof rod-like shaped semiconductor nanocrystals with increased percentageof said second surfactant in the ratio of said surfactants in saidbinary mixture of surfactants; c) adjusting the ratio of said first andsecond surfactants in said binary mixture to control the shape of saidCdSe semiconductor nanocrystals to be formed therein; d) heating saidbinary mixture of phosphorus-containing organic surfactants to atemperature sufficient to permit formation of Group II-VI semiconductornanocrystals therein; e) adding said solution of Cd and Se semiconductornanocrystal precursors to said heated binary mixture of surfactants toform said CdSe semiconductor nanocrystals; and f) subsequently reducingthe temperature of said heated binary mixture of surfactants containingsaid semiconductor nanocrystals to stop said growth of saidsemiconductor nanocrystals.
 32. A rod-like Group II-VI semiconductornanocrystal formed by reacting together a precursor containing one ormore Group II metals selected from the group consisting of Zn, Cd, andHg with a precursor containing one or more Group VI elements selectedfrom the group consisting of S, Se, and Te in a binary mixture ofsurfactants which includes an organo phosphorus-containing surfactantcapable of promoting growth of rod-like semiconductor nanocrystals whena sufficient amount of said surfactant is present in said binary mixtureof surfactants.
 33. The rod-like Group II-VI semiconductor nanocrystalof claim 32 wherein said organo phosphorus-containing surfactantcomprises an organo phosphorus-containing acid surfactant.
 34. Therod-like Group II-VI semiconductor nanocrystal of claim 32 wherein theparticular Group II metal is cadmium (Cd) and the particular Group VIelement is selenium (Se).
 35. The rod-like CdSe semiconductornanocrystal of claim 34 wherein said rod-like CdSe semiconductornanocrystal is hexagonal in cross-section.
 36. A rod shaped CdSesemiconductor nanocrystal formed by reacting together a Cd precursorwith a Se precursor in a binary mixture of phosphorus-containing organicsurfactants containing at least 5 wt. % of an organophosphorus-containing acid surfactant.